In certain critical applications, components must be manufactured from nickel-base superalloys in the form of large diameter ingots that lack significant segregation. Such ingots must be substantially free of positive and negative segregation, and should be completely free of the manifestation of positive segregation known as “freckles”. Freckles are the most common manifestation of positive segregation and are dark etching regions enriched in solute elements. Freckles result from the flow of solute-rich interdendritic liquid in the mushy zone of the ingot during solidification. Freckles in Alloy 718, for example, are enriched in niobium compared to the matrix, have a high density of carbides, and usually contain Laves phase. “White spots” are the major type of negative segregation. These light etching regions, which are depleted in hardener solute elements, such as niobium, typically are classified into dendritic, discrete, and solidification white spots. While there can be some tolerance for dendritic and solidification white spots, discrete white spots are of major concern because they frequently are associated with a cluster of oxides and nitrides that can act as a crack initiator.
Ingots substantially lacking positive and negative segregation and that are also free of freckles are referred to herein as “premium quality” ingots. Premium quality nickel-base superalloy ingots are required in certain critical applications including, for example, rotating components in aeronautical or land-based power generation turbines and in other applications in which segregation-related metallurgical defects may result in catastrophic failure of the component. As used herein, an ingot “substantially lacks” positive and negative segregation when such types of segregation are wholly absent or are present only to an extent that does not make the ingot unsuitable for use in critical applications, such as use for fabrication into rotating components for aeronautical and land-based turbine applications.
Nickel-base superalloys subject to significant positive and negative segregation during casting include, for example, Alloy 718 and Alloy 706. In order to minimize segregation when casting these alloys for use in critical applications, and also to better ensure that the cast alloy is free of deleterious non-metallic inclusions, the molten metallic material is appropriately refined before being finally cast. A conventional technique for refining Alloy 718 (UNS N07718), as well as certain other segregation-prone nickel-base superalloys such as Alloy 706 (UNS N09706) is the “triple melt” technique, which combines, sequentially, vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR). Premium quality ingots of these segregation-prone materials, however, are difficult to produce in large diameters by VAR melting, the last step in the triple melt sequence. In some cases, large diameter ingots are fabricated into single components, so areas of unacceptable segregation in VAR-cast ingots cannot be selectively removed prior to component fabrication. Consequently, the entire ingot or a portion of the ingot may need to be scrapped.
Ingots of Alloy 718, Alloy 706, and other nickel-base superalloys such as Alloy 600 (UNS N06600), Alloy 625 (UNS N06625), Alloy 720, and Waspaloy (UNS N07001), are increasingly required in larger weights, and correspondingly larger diameters for certain emerging applications. Such applications include, for example, rotating components for larger land-based and aeronautical turbines under development. Larger ingots are needed not only to achieve the final component weight economically, but also to facilitate sufficient thermomechanical working to adequately break down the ingot structure and achieve all final mechanical and structural requirements.
The melting of large diameter superalloy ingots accentuates a number of basic metallurgical and processing related issues. For example, heat extraction during melting becomes more difficult with increasing ingot diameter, resulting in longer solidification times and deeper molten pools. This increases the tendency towards positive and negative segregation. Also, larger ingots and electrodes can generate higher thermal stresses during heating and cooling. While ingots of the size contemplated by this invention have been successfully produced in several nickel-base alloys (for example, Alloys 600, 625, 706, and Waspaloy), Alloy 718 is particularly prone to these problems. To allow for the production of large diameter VAR ingots of acceptable metallurgical quality from Alloy 718 and certain other segregation-prone nickel-base superalloys, specialized melting and heat treatment sequences have been developed. One such specialized melting and heat treatment sequence is described in U.S. Pat. No. 6,416,564, entitled “Method for Producing Large Diameter Ingots of Nickel-Base Alloys,” the entire disclosure of which is hereby incorporated by reference herein.
Accordingly, there is a need for an improved method of producing premium quality, large diameter nickel-base superalloy ingots. Spray forming is one method for producing large diameter superalloy ingots. During spray forming, a stream of molten metal is atomized to form a spray of fine molten-metal droplets or particles. The particles are then directed to a collector where they solidify into a coherent, near-fully-dense preform. In certain applications, controlled movement of the collector and atomizer, along with control of the molten metal transport process, allows high quality large preforms to be produced. The spray forming process is capable of producing fine-grained homogeneous microstructures with equiaxed grains and more than 98 percent theoretical density for a wide range of alloys. However, conventional spray forming generally employs fluid impingement atomization techniques, which present a number of drawbacks.
In conventional fluid impingement atomization techniques, either a gas or a liquid is impinged on a stream of a molten metallic material. Impingement using a liquid or certain gases introduces contaminants into the atomized material. Also, given that fluid impingement does not occur in a vacuum environment, even impingement techniques using inert gases can introduce significant levels of impurities into the atomized material. To address this, certain non-fluid impingement atomization techniques that may be conducted in a vacuum environment have been developed. These techniques include, for example, atomization techniques described in U.S. Pat. No. 6,772,961 B2, entitled “Methods and Apparatus for Spray Forming, Atomization and Heat Transfer” (“U.S. Pat. No. 6,722,961”), the entire disclosure of which is hereby incorporated by reference herein. U.S. Pat. No. 6,722,961 describes techniques wherein molten alloy droplets or a molten alloy stream produced by a melting device coupled with a controlled dispensing device are rapidly electrostatically charged by applying a high voltage to the droplets at a high rise rate. The electrostatic forces set up within the charged droplets cause the droplets to break up or atomize into smaller secondary particles. In one technique described in U.S. Pat. No. 6,722,961, primary molten droplets produced by the nozzle of a dispensing device are treated by an electric field from a ring-shaped electrode adjacent to and downstream of the nozzle. Electrostatic forces developed within the primary droplets exceed the surface tension forces of the particles and result in formation of smaller secondary particles. Additional ring-shaped field-generating electrodes may be provided downstream to treat the secondary particles in the same way, producing yet smaller molten particles.
Electron beam atomization is another non-fluid impingement technique for atomizing molten material, and is conducted in a vacuum. In general, the technique involves using an electron beam to inject a charge into a region of a molten alloy stream and/or a series of molten alloy droplets. Once the region or droplet accumulates sufficient charge exceeding the Rayleigh limit, the region or droplet becomes unstable and is disrupted into fine particles (i.e., atomizes). The electron beam atomization technique is described generally in U.S. patent application Ser. No. 11/232,702, entitled “Apparatus and Method for Clean, Rapidly Solidified Alloys”, the entire disclosure of which is hereby incorporated by reference herein.
U.S. Pat. No. 6,722,961 also discloses techniques using electrostatic and/or electromagnetic fields to control the acceleration, speed, and/or direction of molten alloy particles formed by atomization in the process of producing spray formed preforms or powders. As described in U.S. Pat. No. 6,722,961, such techniques provide substantial downstream control of atomized material and can reduce overspray and other material wastage, improve quality, and enhance the density of solid preforms made by spray forming techniques.
Methods of collecting atomized materials as unitary preforms, such as, for example, spray forming and nucleated casting, are well known and have been described in numerous articles and patents. With respect to nucleated casting, specific reference is drawn to U.S. Pat. Nos. 5,381,847, 6,264,717, and 6,496,529 B1. In general, nucleated casting involves atomizing a molten alloy stream and then directing the resultant particles into a casting mold having a desired shape. The droplets coalesce and solidify as a unitary article shaped by the mold, and the casting may be further processed into a desired component. Spray forming involves directing atomized molten material onto a surface of, for example, a platen or a cylinder to form a free-standing preform. Characteristically, the typical solids fraction of the atomized particles significantly differs between spray forming and nucleated casting since, for example, a less fluid and mobile particle is used in the mold-less spray forming process.
In addition, aspects of the initial alloy melting process can present various disadvantages to the overall ingot production process. The alloy melting process involves preparing a charge of suitable materials and then melting the charge. The molten charge or “melt” may then be refined and/or treated to modify melt chemistry, remove undesirable components from the melt, and/or affect the microstructure of articles cast from the melt. Melting furnaces may be powered by means including electricity and the combustion of fossil fuels, and selection of a suitable apparatus is largely influenced by the relative costs and applicable environmental regulations, as well as by the identity of the material being prepared. A variety of melting techniques and apparatus are available today. General classes of melting techniques include, for example, induction melting (including vacuum induction melting), arc melting (including vacuum arc skull melting), crucible melting, and electron beam melting.
Electron beam melting typically involves utilizing thermo-ionic electron beam guns to generate high energy, substantially linear streams of electrons which are used to heat the target materials. Thermo-ionic electron beam guns operate by passing current to a filament, thereby heating the filament to high temperature and “boiling” electrons away from the filament. The electrons generated from the filament are then focused and accelerated toward the target in the form of a very narrow (nearly two-dimensional), substantially linear electron beam. A type of ion plasma electron beam gun also has been used for preparing alloy melts. Specifically, a “glow discharge” electron beam gun described in V. A. Chernov, “Powerful High-Voltage Glow Discharge Electron Gun and Power Unit on Its Base”, 1994 Intern. Conf. on Electron Beam Melting (Reno, Nev.), pp. 259-267, has been incorporated in certain melting furnaces available from Antares, Kiev, Ukraine. Such devices operate by producing a cold plasma including cations which bombard a cathode and produce electrons that are focused to form a substantially two-dimensional, linear electron beam.
The substantially linear electron beams produced by the foregoing types of electron beam guns are directed into the evacuated melting chamber of an electron beam melting furnace and impinged on the materials to be melted and/or maintained in a molten state. The conduction of electrons through the electrically conductive materials quickly heats them to a temperature in excess of the particular melting temperature. Given the high energy of the substantially linear electron beams, which can be, for example, about 100 kW/cm2, linear electron beam guns are very high temperature heat sources and are readily able to exceed the melting temperatures and, in some cases, the vaporization temperatures of the materials on which the substantially linear beams impinge. Using magnetic deflection or similar directional means, the substantially linear electron beams are rastered at high frequency across the target materials within the melting chamber, allowing the beam to be directed across a wide area and across targets having multiple and complex shapes.
Because electron beam melting is a surface heating method, it typically produces only a shallow molten pool, which can be advantageous in terms of limiting porosity and segregation in a subsequently formed ingot. Because the superheated metal pool produced by the electron beam is disposed within the high vacuum environment of the furnace melting chamber, the technique also tends to degas the molten material. Also, undesirable metallic and non-metallic constituents within the alloy having relatively high vapor pressures can be selectively evaporated in the melting chamber, thereby improving alloy purity. On the other hand, one must account for the evaporation of desirable constituents produced by the highly-focused, substantially two-dimensional linear electron beam. This is particularly the case with nickel-base superalloys, which contain various concentrations of elements such as aluminum, titanium, and chromium, for example, which may be volatilized and evaporated from the melt. This may have a substantial deleterious effect on the chemistry and material properties of a subsequently formed ingot. Undesirable evaporation must be factored into production and can significantly complicate alloy production when using electron beam melting furnaces. The electron beam cold hearth melting technique is commonly used in the processing and recycling of reactive metals and alloys. The feedstock may be drip melted by impinging a substantially linear electron beam on an end of a feedstock bar. The melted feedstock drips into an end region of a water-cooled copper hearth, forming a protective skull. As the molten material collects in the hearth, it overflows and falls by gravity into a withdrawal mold or other casting device. During the molten material's dwell time within the hearth, substantially linear electron beams are quickly rastered across the surface of the material, retaining it in a molten form. This also has the effects of degassing and refining the molten material through evaporation of high vapor pressure components. The hearth also may be sized to promote gravity separation between high-density and low-density solid inclusions, in which case oxide and other relatively low-density inclusions remain in the molten metal for a time sufficient to allow dissolution, while high density particles sink to the bottom and become entrapped in the skull.As noted above, many of the known processes for melting, atomizing and forming alloys to produce solid preforms are deficient in one or more respects. Such deficiencies include, for example, the existence of oxides and other contaminants in the final product, yield losses due to overspray, and inherent size limitations. These deficiencies are particularly problematic in the production of certain nickel-base superalloys. Accordingly, there is a need for improved methods and apparatus for melting, atomizing and forming large diameter ingots and other preforms from certain nickel-base superalloys, among other alloys.